The Clinical Endocrinology Branch has a follow-up study of the treatment of differentiated thyroid Carcinoma. In this study patients are often given I-131 treatment. Lithium carbonate inhibits the release of thyroid hormone by the normal thyroid gland. In the thyroid carcinoma project, lithium carbonate is given to patients with differentiated thyroid carcinoma to block the release of I-131 from tumor deposits. Successful blockade of the I-131 release prolongs the residence time of the I-131 in the tumor and thus increases the radiation dose delivered per mC trapped by the tumor. In the study patients are given two 1.5 mCi tracer doses of I-131. Serial Nuclear Medicine images of the activity in the tumor are obtained after the doses and the biological T1/2 of I-131 residence in the tumor is calculated. The initial study, a baseline evaluation, is followed by a second study performed when the patient is taking Lithium. The purpose of the study is to determine the frequency of a lithium effect, and to determine whether a significant increase in the radiation dose delivered to the tumor can be achieved. A preliminary report of this study has been published.